Traditional and present-day variable length packet based switching equipments have not been designed from the “ground-up” to support the requirements of converged multi-service networks.
The most common misconception is that present-day protocol mechanisms, such as DiffServ and MPLS, will support the quality of services (QOS) mechanisms required for real-time present-day premium services. While the 8-bit Differentiated Services Code Point (DSCP) and the 3-bit experimental (EXP) field in the MPLS “shim” header may provide a mechanism to classify high-priority service classes into separate queues, they cannot guarantee or enforce that the QOS and SLA requirements of high-priority traffic are satisfied. It is then up to the switching and QOS mechanisms of the routers/switches to attempt to enforce these requirements.
While there have been significant advances in network processor technologies, the performance bottleneck in the router continues to be the switch fabric. The most common switch architecture prevalent in conventional core switching/routing-equipment is based on either the single stage or the multi-stage CIOQ (Combined Input Output Queuing) crossbar switch fabric is sandwiched between two stages of buffering (input as well as output).
In the single stage cross-bar design, for example, the line cards are connected to a crossbar switch fabric which is generally composed of N2 2×2 switching elements, where N is the number of input/output ports. Incoming packets are buffered at the ingress, and a centralized controller is used to arbitrate all packets waiting at the head of the input queues. The controller schedules all requests from input ports with packet waiting, configures the cross points in the fabric, and finally grants the requests when a path is successfully set up between the input and output ports. The centralized controller, however, presents a performance bottleneck especially for large-scale switches. Such crossbar switch fabric architectures with input queuing, moreover, also suffer from Head-of-line (HOL) blocking that limits the maximum throughput to 58.6% even under uniform traffic scenarios, as described, for example, by M. Karol, M. Hluchyj and S. Morgan in “Input versus Output Queuing on a Space-Division Packet Switch”. IEEE Transactions on Communications, vol. COM-35, No. 12, December 1987.
Another proposal, Virtual Output Queuing (VOQ), wherein each input port maintains virtual queues, one each for the traffic destined to a specific output port, has also been proposed to eliminate HOL blocking at the ingress, as also described by Karol et al (above). Such a scheme involves virtual queues at the input ports. Queuing at the output is made possible with some switch speedup.
There exists, indeed, a considerable body of analysis on the prior CIOQ architectures, demonstrating that CIOQ switches suffer from unpredictable performance—they cannot offer any throughput, fairness or delay guarantees and the worst case is not even known, as concluded by N. McKeown, S. Iyer and R. Zhang in “Routers with a Single Stage of Buffering”, Proc. Of ACM SIGCOMM, August 2002; N. McKeown in “SLIP: A Scheduling Algorithm for Input-Queued Switches”, IEEE Transactions on Networking, vol. 7, No. 2, April 1999; and Y. Tamie and S. C. Chi, “The Symmetric Crossbar Arbiters for VLSI Communication Switches”, IEEE Transactions on Parallel and Distributed Systems, vol. 4, No. 1, pp. 13-27. January 1993.
Though CIOQ routers make up a large fraction of the Internet infrastructure, CIOQ switches actually suffer from the following performance limitations:                Practical CIOQ routers are incapable of providing predictable and bounded latency/jitter required for real-time applications. In fact, there is a wide range between minimum and maximum latency as well as jitter supported through a practical CIOQ switch. The worst-case latency and jitter values are heavily dependent on the traffic pattern and distribution. As a result, CIOQ routers cannot preserve the service characteristics of ATM and Frame Relay services-a requirement to support the converged core.        CIOQ switch architectures show blocking behavior under a variety of real-life non-uniform traffic patterns, even for moderate (35% to 40%) switch fabric utilizations. Latency through the switch can range from 10's of μs to 100's of milliseconds for higher utilizations. Moreover, traffic admittance decisions are made at the inputs to the CIOQ crossbar fabric. Lack of knowledge of output queue state results in sub-optimal decisions at the input resulting in inefficient link utilizations. For instance, traffic could be dropped at the input through Random Early Discard (RED) mechanisms even though some of the outputs do not experience congestion. This causes incoming traffic to be clipped even when egress link bandwidth is available.        
Scheduling algorithms used in CIOQ crossbar switches cannot guarantee bandwidth for different premium traffic classes. Only strict priority can be supported to the switch fabric by the crossbar scheduler, as concluded by N. McKeown, “A Fast Switched Backplane for a Gigabyte Switched Router”, Business Communication Review, December 1997. This implies that services such as VoIP, video, guaranteed IP-VPN and layer to services such as ATM and Frame Relay, each with its own unique bandwidth allocation, cannot be supported through a CIOQ router. The presence of WFQ schedulers at the address line card is not sufficient to support differentiated services if the switch fabric is blocking and the premium packets are not forwarded across the fabric to the egress in time. Thus, it is critical that QOS enforcement and bandwidth management is accomplished in the switch fabric in addition to the line cards.                Conventional equipment based on CIOQ switch architecture has a very noticeable difference between average and worst-case latency as well as jitter. Worst-case latency and jitter are completely unpredictable and can be as high as 100's of milliseconds for certain traffic patterns even at moderate utilizations. Consequently, ATM-grade SLAs cannot be supported for real-time IP services. In addition, burstable services similar to those offered in ATM and Frame Relay is not possible with conventional equipment because the state of the egress queues is not known at the ingress. For instance, packets could be dropped at the ingress to the switches and not allowed to burst even though bandwidth may be available at the egress.        
The above and other limitations of existing data transmission systems and techniques that carriers provide for the myriad of such different types of data traffic, with their costly infrastructures of varied traffic switching networks and routes, including those above-described, moreover, unfortunately enable only a low percent of utilization (often only about 20%) of the transmission link capacity. This is because, as above explained, they are unable to provide the required service quality without degradation, particularly over long distances, of the full and varied characteristics required for particular different types of data packets having different service characteristics and widely different priorities of traffic transmission along the links.
Underlying the present invention, on the other hand, is the discovery of a methodology and technique that provides for normal configurable adaptive output variable length data packet transmission scheduling, and in such a manner that it remarkably enables the full and varied characteristics of all these different types of data packets—all ranging from high priority real-time voice, to financial transactions or the like—to be met on a common transmission link and data transmission flow, while utilizing the full capacity of the link so that there is no waste of bandwidth.
This is achieved, in accordance with the invention, with the aid of novel programmable converged network routers.
In current markets, indeed, there is need for converged networks to reduce capital and operational expenditures. In these converged networks, different types of traffic (like the before-mentioned high priority real-time voice, video, frame relay traffic, etc.) must co-exist with best effort or background traffic on the same link. Thus the converged network routers must preserve traffic characteristics (like bounded latency in general along with bandwidth allocation) of high priority or real-time traffic in the presence of bursty, unpredictable background traffic on the same link. Simultaneously, there is the need for utilizing the link to its full capacity, as earlier stated, so that no bandwidth is wasted on the link.
The invention, accordingly, through its novel programmable converged router control, provides a new algorithmic approach which allows customers to meet the objectives of simultaneous a) preserving of service characteristics for real-time or high-priority traffic, b) bandwidth allocation, and c) maximal link utilization. This is achieved by fine and balanced control of two main characteristics of traffic on the converged network router link: 1) which type of traffic is being transmitted on the link for a given duration, and 2) how much of that traffic is transmitted on the link, all as hereinafter fully detailed.
In accordance with the invention, moreover, the adaptive output scheduler is capable of executing several different quality of service (QOS) algorithms, such as weighted fair queuing (WFQ) strict priority, (SP), round-robin (RR), etc. at an egress port of an electronic data switch fabric (ESF) switch/router that provides differentiated services for various traffic types. The scheduling node with its order of transmission, and the bandwidth allocation management are achieved independently of one another, thereby enabling the router to provide such different services and along a simultaneous data traffic flow somewhat in a kind of “multiplex” fashion. The bandwidth management of the invention, moreover, also implements a full “work-conserving” mode where no bandwidth is ever wasted on an interface and where excess unused bandwidth in one packet data queue may be allocated to another queue that can utilize the same.